Group B Streptococcus (GBS) is a Gram positive bacteria commonly found in the throat and lower intestine of adults, and in the vagina of women. Normally, this organism does not cause disease in the host. During childbirth, however, an infant can become infected with GBS. GBS infections are the leading cause of neonatal morbidity and mortality in the United States, with fatality ratios as high as 50% in untreated cases. In recent years, antibiotics administered during labor have greatly reduced the incidence of neonatal GBS.
Current CDC recommendations call for screening of women for GBS during week 35 to 37 weeks' gestation by culture. Women found to be colonized with GBS are treated with intravenous antibiotics during labor. This approach has reduced the incidence of neonatal infection and fewer patients are provided unnecessary antibiotic treatment. However, the problem of GBS neonatal sepsis has not been eliminated. Colonization with GBS is often transient, so lack of colonization at 35 weeks gestation does not guarantee that there will be no GBS present at week 40. Also, many patients present to healthcare providers first at the time of labor and have not been screened for GBS. The decision to treat with antibiotics in these cases must be made on the basis of risk factors such as gestation <37 weeks, membrane rupture >12 hours, young maternal age, and/or black or Hispanic ethnicity.
Ideally, the determination of GBS colonization would be made during early labor and the laboratory results available within a few hours. Conventional identification of GBS requires culture. As culture may require up to 72 hours for a definitive answer, physicians may provide unnecessary antimicrobics at the time of delivery on an empiric basis. Overuse of antimicrobics may predispose to the development of antimicrobial resistance and add to the risk of adverse reactions including life-threatening anaphylaxis. Rapid antigen tests (e.g., latex agglutination) also have been used to diagnose GBS, but these tests lack sensitivity when compared to culture. In fact, the sensitivities of antigen tests for detecting GBS are so low that the FDA has issued an alert that these types of assays cannot be used to diagnose GBS without culture backup.